1. Field of the Invention
The present invention relates generally to optical sensing techniques and apparatus, and, more specifically, to techniques and apparatus for obtaining a measurement of at least one material in a sample that is substantially free of ambient gas.
2. Description of the Related Art
Optical sensors have long been used to measure the amount of one or more gases in a sample of interest. Optical sensors have been used in a variety of environments, including respiratory gas monitoring. The operation of many optical sensors is based upon a scientific principle known as “absorbance” or “attenuation” of electromagnetic radiation, such as light or infrared radiation.
The wavelengths of electromagnetic radiation that are absorbed by many materials, including gases, liquids, and solid materials, are well known. These absorbed wavelengths of electromagnetic radiation are known as the “absorption peaks” for their respective materials. Some materials absorb electromagnetic radiation of wavelengths that are not absorbed at all or in significant amounts by other materials that may be present in the same sample. For example, the absorption peak for carbon dioxide is centered at a wavelength of about 4.26 μm, whereas nitrogen, oxygen, and other gases that are typically present in monitored respiration do not absorb radiation at this wavelength. Such an absorption peak is useful for monitoring an amount (e.g., concentration, fraction, etc.) of that material in a sample.
The amount of a particular material in a sample may be determined by directing monitoring radiation, including electromagnetic radiation at the absorption peak for that material in a known intensity, into the sample. If the material of interest is present in the sample, the intensity of the wavelength or wavelengths of monitoring radiation that correspond to the absorption peak will decrease, or become “attenuated.” Attenuation may be measured in terms of percent absorbance or optical density (OD). A measure of this decrease in intensity, or “attenuation,” of the monitoring radiation may correspond to the amount of the material in the sample.
Because the intensity of the monitoring radiation may also be decreased by factors other than absorption of the monitoring radiation by the material of interest, reference electromagnetic radiation of a different wavelength, which is not absorbed by the material of interest, and of known intensity may be directed along roughly the same optical path and substantially the same distance as those along which the monitoring radiation travels. The intensity of the reference electromagnetic radiation may be detected by a reference sensor and, thus, the decrease in intensity of the reference electromagnetic radiation may be measured. The measured decrease in intensity of the reference electromagnetic radiation may then be used to determine non-absorption decreases in the intensity of the monitoring radiation. Conventionally, any other decreases in monitoring radiation have been attributed to absorption of some of the monitoring radiation by the material or materials of interest.
Unfortunately, existing technologies do not account for the possible presence of a material of interest in the ambient environment around or within a monitor, or eliminate the material of interest from the ambient environment around or within the monitor. As a result, optical measurements of the material of interest may be somewhat inflated, including not only an amount of the material of interest in a sample, but also the amount of that material present under ambient conditions.